A common cause of missing prop textures is the material files being in a different directory than the one specified in the model's qc file, either because you installed it wrong or it came in an incorrect folder structure. The quickest way to confirm that is to decompile the model and check the qc yourself. You might also want to ask the creator of the prop.

By the way, I hope you don't intend to use these in a TF2 map, because even moderate use of such high-poly models will cause performance to tank hard.

To add to the post above, grass in TF2 is just a 2D sprite with a fairly low-res texture. People in TF2 are generally focussing on combat rather than that one flower or grass(unless they're making Hoovy picnics), so keep it really simple, also for performance reasons.

Of course, if this is for an entirely different game/Source mod, then also do what the post above me says.